1. Field of the Invention
The present invention is directed to a process and apparatus for making crumb rubber from used automobile and truck vehicle tires. More particularly, the present invention is directed to a process and an apparatus of making vehicle tires which involves multiple cryogenic cooling of comminuted rubber particles wherein the cryogenic fluid is recycled.
2. Background of the Prior Art
The daunting number of used tires that must be disposed of annually has not only filled waste disposal sites but, unfortunately, has additionally resulted in indiscriminate discarding of tires creating a major environmental problem. It is for this reason that a great number of processes have been advanced for the disposal of tires in a manner that not only eliminates them as an environmental problem but provides a product that recovers some or all of the cost of such tire disposal.
In one type of disposal scheme a whole plurality of processes have been developed in which used tires are comminuted and the fuel value inherent in the significant volatile content of vehicle tires is recovered. More recently, processes which recover the rubber constituent of vehicle tires, for reuse in the manufacture of rubber-containing articles, have been developed.
Of particular interest are rubber recovery processes which involve the utilization of cryogenic fluids, usually very cold liquid and gaseous nitrogen, to permit the comminution of the tires into small solid particles without attendant melting or significant softening of the rubber.
Although the advance provided by cryogenic cooling of vehicle tires and rubber particles thereof advances the art insofar as it greatly increases the efficiency of the comminuting process, such processes are plagued by two major problems. The first such problem resides in the high cost of utilizing cryogenic fluids. Most of the processes that have been developed utilize fresh sources of cryogenic fluids which, after their employment in cooling and embrittling rubber, are vented to the atmosphere. This venting represents a complete surrender of that charge of the high cost cryogenic fluid.
A second related problem associated with prior art processes is the inability to produce fine sized rubber particles suitable for reuse in the manufacture of rubber articles. Those skilled in the art are aware that very finely divided rubber particles are essential if the crumb rubber product of tire comminution processes can be sold to rubber and rubber application manufacturers. This, in turn, may be the result of the absence of processes that cryogenically treat small enough particles so that they can be further comminuted to the requisite size required for commercial resale. Obviously, this absense may be the result of the expense associated of more than one cryogenic fluid contacting steps.
The above remarks can be better appreciated by reference to illustrative prior art processes which not only set forth the current state of the art but also emphasize their limitations.
U.S. Pat. No. 4,863,106 describes a process and apparatus for the low temperature comminution of tires which includes the recycle of the cryogenic fluid employed therein. In this process tires are shredded into smaller particles which are cooled by contact with a cryogenic gas. Indeed, the process of the '106 patent involves two cooling steps prior to first particlizing tire portions and thereafter comminuting rubber granules. That process also involves recycling the cryogenic cooling gas employed in the two cooling steps for reuse.
A problem associated with the process of the '106 patent is that the cooling gas used in this process cannot provide the degree of cooling necessary to permit the degree of comminution required to produce the fine sized particles essential for production of crumb rubber. In this absence the cost of removing tires from the environment cannot be recouped.
A second disclosure which provides a process and an apparatus for reducing rubber to particles is not particularly directed to the conversion of tires into crumb rubber. Rather, the process of this prior art reference concerns the comminution of scrap rubber, rather than whole tires, into fine sized rubber. It is true, of course, that the scrap rubber may be product of shredded vehicle tires. Be that as it may, U.S. Pat. No. 5,368,240 describes a process wherein rubber particles are loaded on an upwardly moving conveyor belt which is precooled by the vaporization product of a downstream cryogenic liquid bath. That is, the vapors emanating from a downstream cryogenic liquid are directed, by means of a conduit, upwardly by gravity to a precooling chamber. The precooled product of the upwardly moving precooling chamber is thereupon passed through a bath of a cryogenic liquid.
Although the process of the '240 patent avails itself of the vapor from a separately used cryogenic liquid that gas is ultimately vented. Furthermore, the first cooling step does not take particular advantage of the fact that there are two cryogenic fluid contacting cooling steps in that both cooling steps are associated with only one comminution step. The cryogenic gas precooling steps merely enhances the efficiency of the single cryogenic liquid contacting step.
The above illustrations of the prior art emphasize the need in the art for a process and an apparatus that combines a sufficient number of comminution steps preceded by cryogenic liquid cooling to ensure the formation of fine crumb rubber in a manner that insures that the costs and expenses associated conducting these cryogenic cooling steps does not make this process economically infeasible.